Use of graphitic carbon nitrides as solar-light-driven photocatalysts for the reduction of p-nitrobenzoic acid

Abstract

The use of graphitic carbon nitrides (g-C3N4) as photocatalysts for the reduction of p-nitrobenzoic acid (PNBA) under simulated sunlight has been investigated. The photocatalysts were synthesized through the thermal polymerization of melamine and urea. The effects of the g-C3N4 precursor employed in the synthesis, the thermal exfoliation treatment, the addition of sacrificial agents, and the pH conditions were evaluated. It was found that the presence of carboxylic acids as sacrificial electron donors was required to attain the photocatalytic reduction of PNBA, while amines or alcohols did not lead to any activity for this purpose. Furthermore, it was observed that the precursor used in the synthesis of graphitic carbon nitride had a slight influence on the photocatalytic activity, whereas the thermal treatment of the bulk g-C3N4 materials exhibited a favourable effect. The best results were obtained upon addition of oxalic acid at pH = 3 using the carbon nitride exfoliated materials, achieving in these conditions the complete removal of PNBA after ca. 60 min of irradiation. Time resolved profiles of p-aminobenzoic acid (PABA) agree with an initial reduction of PNBA to form this compound, followed by oxidation of PABA by reactive oxygen species formed in the reaction medium.